Two part EEG monitor with databus and method of communicating between the parts

ABSTRACT

A personal wearable EEG monitor comprises a base part ( 1 ) having signal processing means ( 23 ), and an electrode part ( 2 ) with at least two electrodes ( 11, 12 ) for measuring an EEG signal of a person. The electrode part ( 2 ) comprises means for converting the EEG signal into a digital signal. The EEG monitor comprises a databus for transferring data between the base part ( 1 ) and the electrode part ( 2 ) and for providing power from one part to the other. The databus is adapted for application of two electrical wires. The invention further provides a method for communicating between two parts of an EEG monitor.

RELATED APPLICATIONS

The present application is a continuation-in-part of applicationPCT/EP2011053518, filed on Mar. 9, 2011, in Europe and published as WO2011/110582 A1. The present application is a continuation-in-part ofapplication PCT/EP2010/052960, filed on Mar. 9, 2010, in Europe andpublished as WO 2011/110218 A1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to EEG monitors. The invention furtherrelates to an EEG monitor in two parts connected with electrical wires.The invention more specifically concerns an EEG monitor comprising powersupply means. The invention further relates to a method forcommunicating between two parts of an EEG monitor.

EEG is the commonly used abbreviation for Electro Encephalo-Gram, whichis generally speaking a method of electrically monitoring brain activityof a person. Systems for monitoring EEGs have been known for many years.However with the general technological development, EEG monitoringsystems, which may be carried or worn continuously by a person to bemonitored, have been devised.

It is known to measure EEG by placing electrodes on the scalp of aperson, and to record and analyse the EEG signal for various diagnosticpurposes.

2. The Prior Art

A system for such a use is known from WO-A1-2006/047874, which describesmeasurement of brain waves by use of electrodes placed in connectionwith at least one of the ears of the subject, i.e. placed on an outerear part or placed in the ear canal. The measurements are usedparticularly for detecting the onset of an epileptic seizure.WO-A1-2006/047874 also describes the use of electrodes in pairs asdetection and reference electrodes respectively, such a setup being wellknown in the field of electroencephalography.

EEG monitors may also be applied for surveillance of persons havingdiabetes, where blood sugar levels are monitored in order to warnagainst hypoglycemic attacks caused by low blood sugar levels.Hypoglycaemic attacks may lead to unconsciousness and even death. Asystem for such surveillance of an eminent hypoglycaemic attack isdisclosed in WO-A-2006/066577. This is, however, an implantedsubcutaneous system.

WO-A1-2007/047667 describes an ear plug for measuring EEG-signals. Theear plug comprises an exterior shell with electrodes, the shell beingmade of a soft, compressible material. The signals obtained with the earplug are transmitted to external units for processing and monitoring.

Typically, a personal EEG monitor will be made in two parts as mentionedabove, i.e. a base part with signal processing means and an electrodepart with at least two electrodes for measuring the EEG signal of aperson. Preferably the electrode part will be made as small as possible,such that it is easily attached to the skin surface on the head of theperson. The base part often comprises the power supply means, and istherefore larger. The base part would typically be arranged in a lessvisible position. The two parts will be connected through wires. Theelectrodes for measuring the EEG signal are often prepared to bearranged with skin contact in the ear region of a person. The electrodesmay also be capacitive.

Since the electrode part is adapted for being arranged at or in the earor ear region of a person, it will often comprise a receiver or speakerapplied for giving sound or voice messages to the user. This could bewarnings about an imminent hypoglycemic seizure. But the receiver may beapplied for any type of sounds.

The electrode part could also be applied for other types of transducers.Examples of such transducers in the electrode part could be a microphonefor transforming sounds into electrical signals, or a temperature sensoror an accelerometer. Also other transducers could be considered relevantto arrange in the electrode part. The electrical signal from such atransducer needs to be transferred to the signal processing means of thebase part of the EEG monitor, normally by an extra pair of wires, forfurther processing, logging or transmission to a remote device.

One problem in having such a transducer, e.g. a microphone, is that thewires used for transferring the signal from the transducer to the basepart may pick up electromagnetic interference. The electrical signalgenerated in e.g. a microphone may be relatively weak, e.g. 1-5 μV, andtherefore rather sensitive to noise.

This problem is larger when a receiver is arranged in the electrodepart, since the wires supplying the receiver signal, which may be 2 V atpeak level, will be arranged close to the wires transferring the signalfrom the EEG electrodes and e.g. from a transducer. Therefore, there maybe a risk that the receiver signal will induce noise into the wirescarrying the EEG signal and any transducer signal.

US-A1-2004/0116151 describes a databus which can be applied for ahearing aid between a base part and a peripheral component. This databusis described as needing transfer of power, clock and synchronizationsignal.

One problem is that the number of wires should be as low as possible inorder to keep the total diameter of the bundle of wires connecting thetwo parts as small as possible. Each wire is connected both to theelectrode part and to the base part, e.g. through a connector. Thisconnection will take up some space, and will in general be a weak pointin the construction, i.e. there is a risk of losing the electricalconnection at this point. Furthermore the connectors typically appliedare relatively expensive components. Therefore, keeping the necessarynumber of connections to a minimum is to be preferred.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a personal EEG monitor withpower supply means, said EEG monitor comprising a base part havingsignal processing means, an electrode part with at least two electrodesfor measuring an EEG signal of a person, said electrode part havingmeans for converting the EEG signal into a digital signal, and a databusfor transferring data between said base part and said electrode part andfor providing power from one part to the other, said databus beingadapted for application of two electrical wires.

A databus is here understood to be a digital communication line whichcan be set up for communication between different units, suitable forcarrying signals in one or two directions. The databus is a serialdatabus, and is here also understood to be able to transfer power.

A transducer is here understood as a device which can transfer aphysical parameter into an electrical signal in the EEG monitor. Thisdefinition includes an electrode being able to read a voltage potential,such that the potential in some form can be transferred to the signalprocessing means of the EEG monitor.

The electrodes may be prepared for arrangement with skin contact, e.g.in the ear region of the person. The electrode may also be of thecapacitive type where an EEG signal can be detected without a directelectrical contact to the skin.

In an embodiment of the EEG monitor, at least two different states ofthe databus are applied in different time slots, where a first state isfor transfer of power and a second state is for transmission of signalfrom said electrode part to said base part.

In an embodiment of the EEG monitor, at least three different states ofthe databus are applied in different time slots, where a first state isfor transfer of power, a second state is for transmission of signal fromsaid base part to said electrode part, and a third state is fortransmission of signal from said electrode part to said base part. Thisembodiment will often be preferred when a receiver or speaker, e.g. forproviding sound messages to the person wearing the EEG monitor, isapplied.

When separating in time the power transfer from the data transfer therisk of noise problems is reduced. The term different time slots refersto this separation in time of power transfer and data or signal transferin both directions. At the same time the invention facilitates a twowire databus without the need of any further electrical wires.

In a second aspect, the invention provides a personal EEG monitor withpower supply means, said EEG monitor comprising a base part havingsignal processing means, an electrode part with at least two electrodesfor measuring an EEG signal of a person, said electrode part havingmeans for converting the EEG signal into a digital signal, and a databusfor transferring data between said base part and said electrode part andfor providing power from one part to the other, said databus beingadapted for application of two electrical wires; and for the applicationof at least four different states in respective time slots, where afirst state is for transfer of power, a second state is for transmissionof signal from said base part to said electrode part, a third state isfor transmission of signal from said electrode part to said base part;and a fourth state set to low in order for the first state to start witha rising edge.

In an embodiment of the EEG monitor, a fourth state is added which isset to low, i.e. to “0”, in order for the first state for power transferto start with a rising edge. Such a rising edge occurring at a knownplace in the sequence is important in order to interpret the signal onthe databus.

In an embodiment of the EEG monitor, the power supply is arranged in thebase part and a capacitor is arranged in the electrode part, saidcapacitor adapted for being charged during said first state for transferof power, and supplying power in periods where no power is transmittedthrough the databus. There will typically be more space in the base partand therefore more room for power supply, such as a battery.

In an embodiment of the EEG monitor, the first state for transfer ofpower takes up at least 50%, preferably at least 70%, of the time on thedatabus. This has been found to result in a sufficiently small powerloss and a not too large capacitor for supplying power in the rest ofthe time.

In an embodiment of the EEG monitor where a receiver is arranged in theelectrode part, the receiver is connected such that it will not draw anypower in the time where data is transferred on the databus, but only inthe time where power is transferred. This can be achieved byshort-circuiting the receiver during the transfer of data. The advantageof this will be that the receiver will not need to draw power from acapacitor in the electrode part during the time where there is notransfer of power from the base part. This means that the capacitor inthe electrode part can be made much smaller, since it will only need tosupply power to the electronic circuit of the electrode part. A smallercapacitor will also have smaller physical dimensions, whereby theelectrode part can be made smaller. There are possible variations ofthis embodiment, e.g. where the receiver draws power in a smaller partof the time where data is transferred.

In an embodiment of the EEG monitor, the electrode part comprises anelectronic chip, i.e. an integrated circuit (IC), connected with thetransducer, the chip or IC being connected with the databus. The chip isa space efficient way of collecting the necessary circuits, e.g. forhandling the databus communication and power transfer. One circuit is avoltage regulator for the power supply. Another circuit is an analogueto digital converter for converting an analogue signal from the EEGelectrodes and any other transducer into a digital signal. This analogueto digital converter is often a sigma-delta converter.

In an embodiment of the EEG monitor, a clock frequency generator isarranged in either the base part or in the electrode part of the EEGmonitor, and wherein a clock frequency is regenerated, by a clockfrequency regenerator in the part of the EEG monitor without clockfrequency generator. Preferably, this regenerated clock frequency issynchronized with the clock frequency of said clock frequency generator.Usually the clock frequency generator is arranged in the base part ofthe EEG monitor, and often the synchronization is performed by aphase-locked loop.

In an embodiment of the EEG monitor, the electrode part is an ear canalplug comprising at least two electrodes on an external surface, saidelectrodes being arranged for having contact with the ear canal of theuser in order to be able to detect electrical potentials from the personbeing EEG monitored.

In an embodiment of the EEG monitor, the electrode part is connectedwith a transducer for measuring a physical or physiological parameter.Such a transducer could be adapted for measuring temperature, bloodpressure, movement e.g. acceleration, orientation, i.e. whether theperson is lying down. Preferably such transducer is connected to theelectronic module of the electrode part and is prepared for transferringdata to the signal processing means in said base part through the serialdatabus. When the electrode part is arranged as an ear plug, anappropriate transducer for detecting the correct placement of the earplug in the ear canal could also be applied. This could be a capacitivetransducer.

In a third aspect, the invention provides a method for communicatingbetween two parts of an EEG monitor comprising power supply means,comprising the steps of arranging a base part outside the ear canal of auser, said base part having signal processing means, arranging anelectrode part in the ear canal of a person to be monitored, saidelectrode part having EEG electrodes, connecting said electrode partwith said base part through a databus comprising two electrical wiresadapted for transmission of signal to said receiver, and fortransmission of signal from said transducer to said base part, saiddatabus being adapted for providing power supply either from the basepart to the electrode part, or, from the electrode part to the basepart, through said two electrical wires, and applying at least twodifferent states of the two wire databus sequentially in different timespans, where a first state is for transfer of power, and a second stateis for transmission of signal from said electrode part to said basepart.

In an embodiment of the method for communicating between two parts, atleast three different states of the two wire databus are appliedsequentially in different time spans, where a first state is fortransfer of power, a second state is for transmission of signal from thebase part to the electrode part, and a third state is for transmissionof signal from the electrode part to the base part.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be explained in further detailwith reference to the figures.

FIG. 1 illustrates an embodiment where an EEG monitor is provided with adatabus between the base part and the electrode part.

FIG. 2 illustrates the setup of an EEG monitor in three different statesof the databus.

FIG. 3 illustrates the bidirectional digital communication through adatabus, panes (a) through (j) signifying respective signals.

FIG. 4 illustrates different states for controlling the bidirectionaldigital communication, panes (a) through (d) signifying respectivesignals.

FIG. 5 illustrates a phase locked loop circuit applied in an embodimentof the invention.

FIG. 6 illustrates an example of a mechanical layout of an EEG monitor.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the principles of an EEG monitor where the base part 1,often arranged behind the ear, comprises an electronic module 6, and abattery 8. The electronic module 6 comprises signal processing means 23,a clock generator 9 and a controller 24 for controlling thecommunication on the data line or databus 16. The base part may alsocomprise a microphone 3, which can be applied for building a hearing aidcapability into the EEG monitor or for adjusting the sound pressurelevel of any sounds from the receiver 10 in the electrode part 2 to thebackground acoustical noise level. Both for FIG. 1 and the followingFigs. the description is focused on embodiments where a bidirectionaldatabus is applied, and where a receiver or speaker is arranged in theelectrode part.

The electrode part 2 of the EEG monitor comprises an electronic module 7(i.e. an electronic chip or an integrated circuit) and two or more EEGelectrodes 11, 12 for measuring the EEG signal of a person to bemonitored. The electrodes are connected with analogue to digitalconverters 21. The electrode part 2 also comprises a receiver 10.

The electronic module 7 of the electrode part 2 may comprise a digitalto analogue converter 22 for driving the receiver 10, and an analogue todigital converter 21 for digitizing the signal from the EEG electrodes11, 12 and, e.g., any transducer. The digital to analogue converters maybe implemented in the form of delta sigma converters, known from U.S.Pat. No. 5,878,146. A delta sigma converter comprises a delta sigmamodulator and a low pass filter. The delta sigma modulator may bearranged in the base part

For driving the receiver an H-bridge may be applied. An H-bridge isdescribed in WO-A1-2005/076664 and is also illustrated in FIG. 2.

The receiver 10 in the electrode part may be one single unit handlingthe whole frequency spectrum of interest. However, the receiver couldalso be composed of two separate receiver units, one for higherfrequencies and one for lower frequencies.

Two electrical wires 16, 17 or lines are connecting the base part withthe electrode part in the embodiment illustrated in FIG. 1. The twowires making up the databus are for both the power supply and for thedigital communication. A protocol is applied for controlling when poweris transmitted and when data is transmitted in either direction on theserial databus. Different types of protocols may be applied forcontrolling the transmission.

The databus signal may also be sent as a balanced signal on a pair ofwires. This will also reduce the risk of noise influencing the databuscommunication. A balanced pair of wires could be twisted in order tofurther reduce noise influence.

Usually the battery is arranged in the base part, and a voltageregulator is applied for supplying a stable voltage VDD for theelectronic modules. The voltage transferred through the two wires aspart of the protocol needs to charge a capacitor from which power isdrawn during the data transmission on the databus. Often a local voltageregulator 20 in the electrode part is provided.

FIG. 2 shows an example with three main states A, B and C of thedatabus. In the first state A the battery 8 in the base part 1 isconnected through the databus 15, illustrated as a twisted twoelectrical wire connection, to the electrode part 2, where the supplyvoltage will charge the capacitor 25 and power the sound output stage,i.e. the switches 40, 41, 42, 43 in the H-bridge and the receiver 10,e.g. through a voltage regulator. Switches (not shown) in both the basepart and in the electrode part are applied for reconnecting the circuitinto the B state in FIG. 2. In this state the power supply to theelectrode part is disconnected. Instead a transmitter 26 in the basepart is connected through the databus 15 to a data receiver 28 in theelectrode part. During the B state data is transferred from thetransmitter 26 to the data receiver 28. Typically, one bit istransferred during each B state period.

The one or more bit transferred in the B state sets the conditions forthe four switches 40, 41, 42, 43 in the H-bridge in the time duringother states until a new bit or bits have been transferred in the next Bstate. The data receiver 28 should be connected to control logic (notshown) for controlling the switches 40, 41, 42, 43 in the H-bridge. Thecontrol logic will hold the input to the switches until new data havebeen received. If more than one bit is transferred to the electrode partin each B state, the control logic should be set up for storing thesebits and for presenting the correct bit to the input of the switches 40,41, 42, 43 at the appropriate time during the time from one B state tothe next.

In an example indicated in FIG. 2, b_(n) is the level of the one bittransmitted to the data receiver 28 in the B state. The level b_(n) isstored by the control logic, and when shifting from B state to thefollowing C state, the control logic will shift the input on theswitches 40, 41, 42, 43 from b_(n−1) to b. This input b_(n) will be helduntil the end of the next B state where it is shifted to b_(n+1). Theinput b_(n−1) to the switches 40, 41, 42, 43 was transmitted to the datareceiver 28 in the B state previous to the one shown in FIG. 2.

As illustrated in FIG. 2 the switches 40, 41, 42, 43 in the H-bridge areswitched to be open in one diagonal (e.g. 40 and 43) and close in theother (e.g. 41 and 42). This will open for current through the coil ofthe receiver in one direction. When the diagonal where the switches 40,41, 42, 43 are open changes, the direction of the current, and therebythe movement of the membrane, also changes.

The last state shown in FIG. 2 is the C state following the B state whenswitches (not shown) in both the base part and in the electrode part areapplied for reconnecting the circuit into the C state. In the C state atransmitter 29 in the electrode part 2 transmits one or more bitsthrough the databus 15 to a data receiver 27 in the base part. Thesedata transmitted out of the electrode part is the digitized EEG signal.Data from any further transducer will be digitized by an A/D converter21 and packed for transmission in a control unit 18 in the electrodepart.

A further D state where a low bit or a “0” is sent on the databus isoften following the C state, in order to initiate the A state with arising edge. Such a rising edge is used for synchronization between thebase part and the electrode part as described below.

The capacitor 25 will be the power source to the receiver 10, H-bridgeand other power demanding circuits in the electrode part during the B, Cand D states where no power, but only data, is transferred through thedatabus 15. The voltage regulator 20 (see FIG. 1) will ensure that thecorrect voltage is provided in all states. The databus 15 will thus facea relatively low impedance in the A state. In the B state thetransmitter 26 will have low output impedance whereas the data receiver28 will have high impedance. In the C state the transmitter 29 will havelow output impedance whereas the data receiver 27 will have high inputimpedance.

In practice the capacitor 25 may be implemented as two capacitors inparallel (not shown). This would facilitate that one of these twocapacitors could be applied for providing power supply to the H-bridgein the B and C state, and the other one of these two capacitors could beapplied for providing power supply to either the data receiver 28 in theB state or to the transmitter 29 in the C state.

In an embodiment where the receiver 10 or speaker is connected such thatit will not draw any power in the time where data is transferred on thedatabus, but only in the time where power is transferred, the fourswitches 40, 41, 42, 43 in the H-bridge should be operated differently.The control logic controlling the switches 40, 41, 42, 43 in theH-bridge will then hold the input to the switches as described aboveonly in the state where power is transferred, i.e. state A in theexample. In the other states the receiver 10 could be short circuited inorder not to draw any power from the capacitor 25. Short circuiting thereceiver 10 may be achieved by opening switches 40, 41 simultaneouslyand closing switches 42, 43 simultaneously. It could also be opposite,i.e. closing switches 40, 41 and opening switches 42, 43.

FIGS. 3 and 4 shows one example on how the power supply and thecommunication through a two wire bidirectional serial databus 16 couldbe handled. In FIG. 3 pane a, a 32 MHz clock frequency generated in thebase part 1 is shown. A corresponding 32 MHz clock frequency isgenerated in the electrode part 2 by application of a phase-locked loop(PLL) circuit 19 (see FIG. 5). The PLL 19 regenerates the 32 MHz clockfrequency by application of the databus signal. The PLL continuouslyadjusts the synchronization between the two 32 MHz clock frequencies, byapplication of rising edges in the data line signal. When the clockgenerator 9 is arranged in the base part, as in this example, the PLL isarranged in the electrode part. This synchronization is important forthe proper functioning of the communication between the base part 1 andthe electrode part 2.

The 32 MHz clock frequency is to be regarded as an example. Also otherclock frequencies can be applied.

As illustrated in FIG. 3 pane a, the 32 MHz clock cycles can be dividedinto four different states (see top of FIG. 3) called A, B, C and D. Instate A power is transferred, preferably from the base part to theelectrode part. In state B data is transferred from the base part to theelectrode part. This would typically be the electrical signal to thereceiver for the receiver to generate the acoustic signal. In state Cdata is transferred from the electrode part to the base part. Such datais the digitized signal from the EEG electrodes and maybe othertransducers in the electrode part. The state D is always low or “0” suchthat the state A will start with a rising edge. This gives a rising edgefor every cycle where the rising edges have a well defined timeinterval. These rising edges are then applied for synchronization of theclock frequency between the base part and the electrode part. The orderof the suggested states may be different. The state A could also bedivided into two, or more, parts, separated by interchanging B and Cstates. It is also possible to add further states with other purposes inbetween the described states.

FIG. 3 panes b and c show an example on sending one bit from the basepart to the electrode part, where a “0” is sent in FIG. 3 pane b and a“1” is sent in FIG. 3 pane c. In both FIG. 3 pane b and in FIG. 3 panec, a “0” is sent out of the electrode part.

FIG. 3 panes d and e show an example on sending one bit from theelectrode part to the base part, where a “0” is sent in pane d and a “1”is sent in pane e. In both FIG. 3 panes d and e, a “0” is sent out ofthe base part.

FIG. 3 pane f shows the resulting signal on the bidirectional databus,where the dashed lines indicate that the signal can follow one of thetwo possible routes, resulting in either a “0” or a “1” being sent. Thisresulting signal on the databus is a summation of signals from FIG. 3pane b or c, and FIG. 3 pane d or e. In the example there will be arising edge, indicated by an arrow in FIG. 3 pane f, in the databussignal for every 32 rising edges in the 32 MHz clock frequency. Thismeans that the signal on the databus must go low before this risingedge, which is also the case in the databus signal shown in FIG. 3 panef, due to the D state. A change in the databus signal level only occurson rising edges of the 32 MHz clock frequency.

The mentioned rising edges in the data line signal, indicated with anarrow in FIG. 3 pane f, are applied for the PLL to synchronize the clocksignals between the base part and the electrode part.

FIG. 4 shows signals applied in the synchronization of the clockfrequency. FIG. 4 pane a further illustrates the counting of phases by aphase counter. A phase counter is present in both the base part and inthe electrode part. The phase counter is part of a control means 18 ofthe electrode part. The two phase counters are synchronized by the PLLvia rising edges on the databus. The phase counter starts on 1 on arising edge of the databus signal and increments by one for each risingedge on the 32 MHz clock until 32. After 32 the phase counter startsfrom 1 again. The phase counters could also be incremented by half byidentifying the falling edges on the 32 MHz clock.

The phase counters are applied for identifying the states A where poweris to be transferred, and the states B and C where either the base partor the electrode part is sending data out.

FIG. 4 pane b repeats the 32 MHz clock frequency, and FIG. 4 pane crepeats the databus signal, both for ease of comparison in FIG. 4. It isseen from FIGS. 3 and 4 that the state A is active in the phase 1-29,the state B is active in the phase 30, the state C is active in thephase 31 and the state D, where a “0” is transmitted, is active in thephase 32. The phase count is also applied for shifting between thedifferent setups illustrated in FIG. 2 for the different states. Thedifferent phases with the different states are regarded as differenttime slots.

The rising edge between the state D and the state A is intended forsynchronization of the clock frequency in the base part and in theelectrode part. This rising edge is illustrated with arrows in FIG. 3pane f and in FIG. 4 pane c. A different rising edge will occur betweenstate B and C every time a “0” is sent out by the base part followed bya “1” sent out by the electrode part. In order to discriminate betweenthese two rising edges, the control unit 18 of the electronic module 7of the electrode part 2 is arranged for generating a signal to beapplied for this discrimination. This signal is called Trig_on and isillustrated in FIG. 4 pane d.

The Trig_on signal is set to “1” (or high), when the phase equals 32or 1. The Trig_on signal is set to “0” (or low), when the phase is from2 to 31. At least Trig_on should be low in phase 30 and 31.

FIG. 5 shows an example of the phase locked loop (PLL) circuit 19applied for synchronizing the 32 MHz clock frequency between the basepart and the electrode part by application of rising edges marked witharrows in FIG. 3 pane f and FIG. 4 pane c. The data line signal goes toan AND operator 30 together with the Trig_on signal. The output of theAND operator 30 will thus only go high for the rising edges of the dataline signal, marked with an arrow, and not for the rising edge when a“0” is sent out of the base part followed by a “1” sent out of theelectrode part (see FIG. 4 pane c and d). This is because the Trig_onsignal is high at the data line rising edge marked with an arrow, whileit is low when sending signal bits out of the base part or out of theelectrode part.

The signal from the AND operator 30 is the reference input to the phasefrequency detector (PFD) 31. The other input to the PFD 31 is thefeedback from the voltage controlled oscillator (VCO) 32 through adivider 33. The two outputs QA and QB of the PFD 31 control a firstswitch 34 and a second switch 35 through a train of pulses. A firstconstant current generator 36 and a second constant current generator 37will either charge or discharge a capacitor 38, thereby determining theinput voltage to the VCO 32. The two current generators 36, 37 usuallygenerate the same current. A pulse on QA will close the first switch 34connected with QA, whereby the first constant current generator 36 willbe charging the capacitor 38. A pulse on QB will close the second switch35 connected with QB, whereby the second constant current generator 37will be discharging the capacitor 38.

When the two signals on the inputs of the PFD 31 are synchronized orlocked, the length of the pulses QA and QB are the same and the voltageon the VCO 32 input remains unchanged. If the two signals on the inputsof the PFD 31 are out of synchronization, the pulses on one of theoutputs QA and QB of the PFD 31 become longer than the pulses on theother output, thereby either charging or discharging the capacitor 38.This will adjust the input voltage on the VCO 32 to a level where theoutput frequency of the VCO is synchronized with the databus signal.

When starting up the databus, especially in the example of thebidirectional databus, e.g. when turning on the EEG monitor, or whenresetting the databus, the controller 18 should wait for the PLL tolock, i.e. for the two 32 MHZ frequencies to become synchronized. Thisis the case when the lengths of the pulses QA and QB are the same orapproximately the same. When this happens, the electrode part will bewaiting for a rising edge on the data line. When the controller 18detects a rising edge on the data line, the phase counter is set to 1.From this point in time the phase counter will continue as shown in FIG.4 pane a, and as described above. In order for this start up procedureto function properly, the situation in FIG. 3 pane i should be avoided,i.e. a “0” from the base part followed by a “1” from the electrode partshould be avoided during start up in order not to get any other risingedge which could disturb the synchronization. This means that thedatabus signal initially has to look like the signal in FIG. 3 panes g,h or j.

Resetting the databus, and subsequent application of the above start-upprocedure, can be initialized if the connection at one or more lines orwires is temporarily lost. Such a temporary loss of connection can bedetected by the control circuit 18 of the electronic module 7 in theelectrode part. This could be done by checking the voltage over thecapacitor 38 in the PLL 19 (see FIG. 5). The rising edges of the databussignal stops, this voltage will fall towards zero, and when the controlcircuit 18 detects this, the electrode part should stop sending data onthe databus, and at the same time the above start-up procedure should beinitialized. The control circuit 18 may also be set up for detecting anytemporary loss of connection on the power supply wires.

A specific code may be applied for confirming that the clock frequenciesare properly synchronized. This code, or a different code, could also besent with specific time intervals to confirm that the communication isfunctioning as scheduled. If this code stops, or the time intervals arenot properly followed, a reset procedure could also be initialized. Suchcode will need to be sent as part of the signals sent out of the basepart or out of the electrode part arranged at specific times in thesequence of data signals.

In the above example of the data communication, one cycle of the clockfrequency is applied for sending one bit from the base part to theelectrode part and one bit from the electrode part to the base part. Thedata communication could be arranged in many other ways. Other optionswithin the embodiments of the invention could be to send e.g. 2 or 4bits from the base part followed by the same, or a different, number ofbits sent from the electrode part to the base part. The advantage ofonly sending one bit at a time is that the capacitor needed in theelectrode part for holding the supply voltage can be relatively smallersince the time in which the supply voltage needs to be held, without thecapacitor receiving extra charge, will be relatively shorter. The numberof bits sent in each of the two directions does not have to be the same.This could depend on the needs of the databus and the one or moretransducers in the electrode part.

Also the clock frequency will influence the necessary size of thecapacitor. With a 32 MHz clock frequency, power will be transferred inthe fraction 29/32 of time according to the example above where data issent out of the base part at 1 Mbit/s and data is sent out of theelectrode part at 1 Mbit/s. This means that the capacitor 25 only needsto hold the supply voltage in 3/32 of a microsecond. If the clockfrequency was 4 MHz and the demands for data transfer were the same, thecapacitor would need to hold the supply voltage for ¾ of a microsecond.At the same time power would only be transferred in the fraction ¼ oftime. This means that the capacitor should be larger and that thecurrent running through the databus while transferring power would needto be higher in order to supply the necessary charge.

A higher current during the power supply period, i.e. state A, will leadto a higher power loss compared to the power loss at a lower current.

When the time fraction where no power is transferred is increased, thesize of the capacitor 25 needs to be increased, in order for thecapacitor to hold enough charge to be able to supply power in the timewithout power supply. Larger capacity also means physical largerdimensions of the capacitor. Due to the need for a small electrode part,a small capacitor, and thus a relatively high clock frequency will oftenbe preferred.

A higher frequency will, however, also lead to a higher dynamicefficiency loss in the p-n junctions of the control circuit. This powerloss is caused by charging the capacitive load of logic gates. For thedatabus alone the actual frequency causing this power loss is lower thanthe clock frequency, since the databus will be on the same level duringthe cycles of the A state. The number of shifts between “0” and “1” willtherefore often be considerably lower than the controlling clockfrequency, i.e. 32 MHz in the example. Thereby, the dynamic efficiencyloss is also reduced.

1 Mbit/s should be sufficient for supplying the receiver 10 with anelectrical sound signal of the necessary quality. For electrodes 11, 12in the electrode part 2 the signal is digitized by A/D converters 21 andthis may result in a signal of less than around 2 Mbit/s. The EEGsignals, which may comprise several inputs from different electrodes,will usually be pre-processed in the electrode part and thereby reducedto approximately 600 kbit/s or less. Signals from other transducers inthe electrode part will also be pre-processed and transferred as part ofthis signal. A signal at this rate can easily be transmitted through thedatabus of the above example. The preprocessing is a decimation of thesignal by a reduction of the sampling frequency and a low pas filtering,whereby high frequency, quantification noise is removed.

In the embodiment where the receiver 10 or speaker is connected suchthat it will not draw any power in the time where data is transferred onthe databus, but only in the time where power is transferred, themaximum acoustic output power from the receiver 10 will be reducedslightly. In the example with a 32 MHz clock frequency where power istransferred in the fraction 29/32 of the time the reduction in maximumacoustic output power from the receiver 10 will be 3/32 or approximately1 dB.

When adding further transducers to the electrode part, where data needsto be transferred through the databus to the base part, furtherbandwidth of the databus is necessary. Depending on the type of thesetransducers the amount of data to transfer may vary significantly. Ifthe transducer is a thermometer or an accelerometer for detection ofmovements, the necessary amount of data for transfer may be relativelylimited, whereas when the transducer is a microphone more data need tobe transferred.

When a number of transducers are comprised in or connected with theelectrode part, the data from these may be collected by the electronicmodule 7 of the electrode part and packaged into a format suitable forsending via the databus together with the digitized EEG signal.

FIG. 6 shows an example of an EEG monitor with the base part 1 preparedto be arranged behind the ear with the electrode part 2 prepared to bearranged in the ear canal of the person who needs to have the EEG signalmonitored. The base part and the electrode part are connected by twoelectrical wires 14. The electrode part 2 is here illustrated as havingtwo electrodes 11, 12, but will often have more electrodes, e.g. three,four or five. With a higher number of electrodes it may be possible toselect the EEG signal from those electrodes having the best contact orfor other reasons receiving the best EEG signal. Also differences in theEEG signal between different sets of electrodes may be applied in theanalysis of the EEG signal.

The electrode part is formed as an ear plug shaped to fit the ear canalof the person who needs to wear the EEG monitor. By shaping the ear plugto the ear canal of the individual, the electrode part will be lessannoying to wear, and it is ensured that the electrodes will always beplaced at the same spot in the ear canal.

The electrode part will usually be provided with a through going opening13 in order for sound to pass into the ear canal without beingobstructed. The speaker arranged in the ear plug may apply this opening13 or a different opening for supplying sound into the inner part of theear canal.

The electrodes 11, 12 illustrated in FIG. 6 may be of any type, e.g.being based on obtaining an electrical connection to the skin or beingof the capacitive type where no electrical connection is obtained, butthe EEG signal is measured through a capacitive coupling to the skinsurface.

We claim:
 1. A personal EEG monitor, comprising a base part having asignal processor, an electrode part with at least two electrodes formeasuring an EEG signal of a person, said electrode part having ananalog-to-digital converter for converting the EEG signal into a digitalsignal, and a two-wire databus for transferring data between said basepart and said electrode part over two wires with said base part andelectrode part located adjacent one ear, and for providing power oversaid two wires from one part to the other; and for the application of atleast four different states in respective time slots, where a firststate is during a first period of time and is for transfer of power, asecond state is during a second period of time and is for transmissionof signal from said base part to said electrode part, a third state isduring a third period of time and is for transmission of signal fromsaid electrode part to said base part; and a fourth state is during afourth period of time and is set to low in order for the first state tostart with a rising edge, with no power being transferred between saidbase part and electrode part during said second and third states, andwherein data to be transferred includes at least first and secondvalues, and said databus represents said first value with a first signallevel and represents said second value with a second signal level.
 2. Apersonal EEG monitor, comprising a base part having a signal processor,an electrode part with at least two electrodes for measuring an EEGsignal of a person, said electrode part having an analog-to-digitalconverter for converting the EEG signal into a digital signal, and atwo-wire databus for transferring data between said base part and saidelectrode part over two electrical wires with said base part andelectrode part located adjacent one ear, and for also providing powerfrom one part to the other over said two wires, wherein data to betransferred includes at least first and second values, and said databusrepresents said first value with a first signal level and representssaid second value with a second signal level different from the firstsignal level.
 3. The EEG monitor according to claim 2, wherein twodifferent states of the databus are applied in different time slots,including at least a first state for transfer of power without transferof data, and a second state for transmission of signal from saidelectrode part to said base part without transfer of power.
 4. The EEGmonitor according to claim 3, wherein said databus is configured suchthat said first state for transfer of power takes up at least 50% of thetime on the databus.
 5. The EEG monitor according to claim 4, whereinsaid first state for transfer of power takes up at least 70% of the timeon the databus.
 6. The EEG monitor according to claim 2, wherein atleast three different states of the databus are applied in differenttime slots, where a first state is for transfer of power, a second stateis for transmission of signal from said base part to said electrode partwithout transfer of power, and a third state is for transmission ofsignal from said electrode part to said base part without transfer ofpower.
 7. The EEG monitor according to claim 2, wherein said electrodepart includes a capacitor charged during said first state and forsupplying power to components of said electrode part during said secondand third states, said components including a receiver which is poweredfrom said two-wire databus and not from said capacitor.
 8. The EEGmonitor according to claim 6, wherein said electrode part includes areceiver, and wherein supply of power to said receiver is disconnectedduring at least one of said second and third states.
 9. The EEG monitoraccording to claim 2, wherein a power supply is arranged in said basepart and a capacitor is arranged in said electrode part, said capacitorbeing adapted for being charged during a first state for transfer ofpower, and for supplying power in periods where no power is transmittedthrough the databus.
 10. The EEG monitor according to claim 9, whereinsaid electrode part comprises a receiver for providing a sound signal tosaid person.
 11. The EEG monitor according to claim 10, wherein saidreceiver is connected such that it will not draw any power in the timewhere data is transferred on the databus.
 12. The EEG monitor accordingto claim 2, wherein said electrode part is adapted to be arranged in theear canal of the person to be monitored.
 13. The EEG monitor accordingto claim 2, wherein said electrode part comprises an electronic chipconnected with the at least two electrodes, said electronic chip beingconnected with said databus.
 14. The EEG monitor according to claim 13,wherein said electronic chip comprises a power supply and a voltageregulator for the power supply.
 15. The EEG monitor according to claim13, wherein said analog-to-digital converter is located on saidelectronic chip.
 16. The EEG monitor according to claim 15, wherein saidanalog-to-digital converter is a sigma-delta converter.
 17. The EEGmonitor according to claim 2, comprising a clock frequency generatorarranged in either the base part or in the electrode part of the EEGmonitor, and a clock frequency regenerator in the part of the EEGmonitor without the clock frequency generator.
 18. The EEG monitoraccording to claim 17, wherein said clock frequency regenerator issynchronized with the clock frequency of said clock frequency generator.19. The EEG monitor according to claim 18, comprising a phase-lockedloop for synchronizing the clock frequency regenerator.
 20. The EEGmonitor according to claim 17, wherein said clock frequency generator isarranged in said base part of the EEG monitor.
 21. The EEG monitoraccording to claim 2, wherein the electrode part is adapted to bearranged in the ear canal of the one ear of the person to be monitoredand comprises a receiver, and said EEG monitor comprises soundamplification means and at least one microphone.
 22. The EEG monitor ofclaim 2, wherein said two wires are used for transmission of data fromsaid base part to said electrode part and from said electrode part tosaid base part, and for transfer of power.
 23. The EEG monitor accordingto claim 2, wherein said two-wire data bus transfers power between saidbase part and said electrode part during a time when no data is beingtransferred between said base part and electrode part.
 24. The EEGmonitor according to claim 2, wherein each of said two wires provides anunbroken electrical path between said base part and electrode part. 25.The EEG monitor according to claim 2, wherein there is no electricalconnection between said base part and electrode part that does not passthrough said two wires.
 26. A method for operating the EEG monitor ofclaim 2, comprising the steps of: arranging said base part outside theear canal of the one ear, arranging said electrode part in the ear canalof the person, and connecting said electrode part with said base partthrough said databus.
 27. The method according to claim 26, furthercomprising a step of applying at least two different states of the twowire databus sequentially in different time spans, where a first stateis for transfer of power, and a second state is for transmission ofsignal from said electrode part to said base part.
 28. The methodaccording to claim 26, comprising applying at least three differentstates of the two wire databus sequentially in different time spans,where a first state is for transfer of power, a second state is fortransmission of signal from said base part to said electrode part, and athird state is for transmission of signal from said electrode part tosaid base part.